Efforts have been made in the past to include fusible elements in stem operated valves so that leakage around the stem could be prevented by a metal-to-metal stem seal when the valve was subjected to fire. The H. Allen U.S. Pat. No. 3,788,600, the A. S. Volpin U.S. Pat. No. 2,647,721 and the W. M. Kelly et al U.S. Pat. No. 3,842,853 are examples of such prior structures.
The Allen Patent discloses a gate valve with a two-piece stem, the outer section of which telescopes with respect to the inner section. The outer section has a metal sealing shoulder which is held apart from a metal seat, located in the cap, by a fusible member. In the event of a fire, a metal-to-metal seal is formed between the stem shoulder and cap seat. This will effectively contain the line fluid; however, the cap must be made excessively large since it will be exposed to the high pressure of the line fluid.
The A. S. Volpin Patent discloses a valve with a fusible element supporting the bearing so that a shoulder on the stem engages a seat on the interior of the bonnet when the fusible element melts due to fire. This structure positions the stem packing on the outer end of the stem where it is more subject to heat than the fusible element and could fail before the fusible element to allow escape of fluids around the stem which are flowing through the valve and which fluids may feed the fire or complicate the fire fighting. Further, Volpin makes no provision for draining the melted material from the bonnet. Also, Volpin, as does Allen, requires the cap to be strong enough to contain the high pressure line fluid.
The W. M. Kelly et al Patent discloses a heat responsive safety valve in which the gate is held in open position by fusible material at the outer end of the stem retaining the stem from axial movement. The occurrence of a fire melts the fusible material and responsive to a spring load the stem and gate move to close passage through the valve. There is no provision for a metal-to-metal sealing around the stem inward of the O-ring stem seal. Failure of such O-ring seal could lead to the leakage of fluids from the valve around the stem even though the gate had moved to closed position.